Gas turbine engines typically include turbine stages, which deliver air to a compressor section. The compressed air is delivered downstream to a combustion section. The energy of a gas turbine engine is generated by burning a fuel-air mixture, which is characterized by a high excess of oxygen i.e. lean burning. During the combustion a stream of hot expanding gas forms. The hot expanding gases are directed from the combustion chambers through the blades of a turbine. As the air flows through this area, velocity is increased accompanied by a decrease in air pressure. The rotary power of the turbine can rotate the compressor to replenish air supply. The gases still possessing energy are discharged to the atmosphere through a nozzle that accelerates the gas stream. Thus thrust of the aircraft can be produced by the escaping gas stream.
As air traffic grows, a development is needed to reduce aircraft engine emissions in order to minimize the environmental impact in the troposphere and at high altitudes as in the lower stratosphere, where the main cruising routes are. The Intergovernmental Panel on Climate Change (IPCC 2007) recommends that developed countries need to reduce emissions to 25-40% within the next 10 years, and indicate a sustainability goal for 2050 of about 85% reduction in carbon dioxide (abbreviated hereinafter with CO2) emissions from 1990 levels in order to stabilize atmospheric greenhouse gases. Aircrafts emit CO2 and nitrogen oxides (abbreviated hereinafter with NOx where x is an integer) and therefore contribute to greenhouse gases. The fact that aircrafts are already included in the emission trading scheme represents an incentive to reduce these emissions. The CO2 emission issue is mainly related to engine efficiency improvement. On the other hand, higher engine efficiency at a constant rate of power leads also to a decrease in NOx emissions. The role of NOx is important in the formation of tropospheric ozone and acid rain. Therefore it is essential to exploit technical possibilities for limiting NOx emissions.
A distinction is made between thermal NO, which is formed from atmospheric nitrogen at high gas temperatures prevailing in the combustion section of a gas turbine, and fuel-NO, which is formed from the nitrogen contained in the fuels at lower temperatures. Under fuel lean conditions and at high temperatures molecular nitrogen (N2) and oxygen (O2) in the combustion air dissociate and produce nitric oxide emissions by a series of reactions wherein the three principal are:N2+O→NO+N  (1)N+O2→NO+O  (2)N+OH→NO+H  (3)
Besides primary measures for emission reduction, which are combustion related, secondary measures as exhaust gas treatments serve to decompose NOx already formed. However, in the aviation sector due to the fact that the thrust of the core flow of the engines is essentially required for aircraft propulsion in high altitudes, the use of a secondary denitrification technology as a catalyst located in the exhaust flow is not possible. Therefore there is an demand for secondary measures applicable to the high temperature processes in turbojet engines without disturbing the jet propulsion.